The present invention relates generally to the field of trenchless underground boring and, more particularly, to a system and process for acquiring positional and orientation data on an underground boring tool.
Utility lines for water, electricity, gas, telephone and cable television are often run underground for reasons of safety and aesthetics. In many situations, the underground utilities can be buried in a trench which is then back-filled. Although useful in areas of new construction, the burial of utilities in a trench has certain disadvantages. In areas supporting existing construction, a trench can cause serious disturbance to structures or roadways. Further, there is a high probability that digging a trench may damage previously buried utilities, and that structures or roadways disturbed by digging the trench are rarely restored to their original condition. Also, an open trench poses a danger of injury to workers and passersby.
The general technique of boring a horizontal underground hole has recently been developed in order to overcome the disadvantages described above, as well as others unaddressed when employing conventional trenching techniques. In accordance with such a general horizontal boring technique, also known as microtunnelling or trenchless underground boring, a boring system is situated on the ground surface and drills a hole into the ground at an oblique angle with respect to the ground surface. Water is typically flowed through the drill string, over the boring tool, and back up the borehole in order to remove cuttings and dirt. After the boring tool reaches a desired depth, the tool is then directed along a substantially horizontal path to create a horizontal borehole. After the desired length of borehole has been obtained, the tool is then directed upwards to break through to the surface. A reamer is then attached to the drill string which is pulled back through the borehole, thus reaming out the borehole to a larger diameter. It is common to attach a utility line or other conduit to the reaming tool so that it is dragged through the borehole along with the reamer.
In order to provide for the location of a boring tool while underground, a conventional approach involves the incorporation of an active beacon, typically in the form of a radio transmitter, disposed within the boring tool. A receiver is typically placed on the ground surface and used to determine the position of the tool through a conventional radio direction finding technique. However, since there is no synchronization between the beacon and the detector, the depth of the tool cannot be measured directly, and the position measurement of the boring tool is restricted to a two dimensional surface plane. Moreover, conventional radio direction finding techniques have limited accuracy in determining the position of the boring tool. These limitations can have severe consequences when boring a trenchless underground hole in an area which contains several existing underground utilities or other natural or man-made hazards, in which case the location of the boring tool must be precisely determined in order to avoid accidentally disturbing or damaging the utilities.
Recently the use of ground penetrating radar (GPR) for performing surveys along trenchless boring routes has been proposed. Ground-penetrating-radar is a sensitive technique for detecting even small changes in the subsurface dielectric constant. Consequently, the images generated by GPR systems contain a great amount of detail, much of it either unwanted or unnecessary for the task at hand. A major difficulty, therefore, in using GPR for locating a boring tool concerns the present inability in the art to correctly distinguish the boring tool signal from all of the signals generated by the other features, such signals collectively being referred to as clutter. Moreover, depending on the depth of the boring tool and the propagation characteristics of the intervening ground medium, the signal from the boring tool can be extremely weak relative to the clutter signal. Consequently, the boring tool signal may either be misinterpreted or undetectable.
It would be desirable to employ an apparatus for determining a location and an orientation of an underground boring tool with higher accuracy than is currently attainable given the present state of the technology.
The present invention relates to an apparatus and method for determining a location and an orientation of an underground boring tool by employment of a radar-like probe and detection technique. The boring tool is provided with a device which generates a specific signature signal in response to a probe signal transmitted from above the ground. Cooperation between the probe signal transmitter at ground level and the signature signal generating device provided at the underground boring tool results in accurate detection of the boring tool location and, if desired, orientation, despite the presence of a large background signal. Precision detection of the boring tool location and orientation enables the operator to accurately locate the boring tool during operation and, if provided with a directional capacity, avoid buried obstacles such as utilities and other hazards.
The signature signal produced by the boring tool may be generated either passively or actively, and may be a microwave or an acoustic signal. Further, the signature signal may be produced in a manner which differs from that used to produce the probe signal in one or more ways, including timing, frequency content, information content, or polarization.
In accordance with one embodiment, surveying the boring site, either prior to or during the boring operation, provides for the production of data associated with the characteristics of the ground medium subjected to the survey. The ground characteristic data acquired during the survey may be correlated with historical data which relate ground types to boring productivity, hence enabling estimates of boring productivity and overall cost to be made for the site subjected to the survey. Accurate surveys of planned boring pathways can be made and the position of the boring tool accurately measured during a boring operation for contemporaneous or subsequent comparison with the planned pathway. The direction of the boring tool may be adjusted in response to the measured position in order to maintain the boring tool along the planned pathway.